Introduction

The facial and trigeminal nerves carry out the innervation of the face. Whereas the facial nerve provides the motor innervation to the facial expression muscles, the trigeminal nerve is mainly the source of sensory stimulation. Also, it supplies motor function to the mastication muscles. The sensory map of the face is further divided into the three main branches of the trigeminal nerve: ophthalmic, maxillary, and mandibular. The ophthalmic nerve provides sensory and parasympathetic innervation to the eye region, and the maxillary nerve innervates mainly below the eye extending to the upper lip. Finally, the third division of the trigeminal nerve, the mandibular nerve, supplies sensory innervation to the territory correlating with the mandibular bone. This nerve gives rise to a terminal branch: the mental nerve responsible for providing sensory innervation to the oral mucosa, lower lip, and the skin of the chin ventral to its foramen [1].

Structure and Function

As the trigeminal nerve exits the pons, it will divide into the ophthalmic nerve, the maxillary nerve, and the mandibular nerve. The mandibular nerve leaves the skull through the foramen ovale to give rise to a meningeal nerve called the nervus spinosus, which re-enters the skull through the foramen spinosum, and a nerve for the medial pterygoid muscle [2]. After this, the main trunk of the mandibular nerve further divides into an anterior and posterior branch. Next, the posterior trunk splits into three branches: the auriculotemporal, lingual, and the inferior alveolar nerve. The inferior alveolar nerve enters the mandible through the mandibular foramen and travels throughout the mandibular canal anteriorly until reaching the mental foramen. It receives the name of the mental nerve once it exits this foramen [3][4].

The mental nerve will provide sensory innervation to the anterior territory of the buccal mucosa, lower lip, and the skin of the chin ventral to its foramen. Once leaving the mental foramen, it usually divides into three branches beneath the depressor anguli oris muscle. One descends toward the chin to give sensory innervation to the skin of the anterior chin. The other two branches will ascend to the lower lip and buccal gingivae around the premolar teeth [3][5]. The sensory innervation of the mental nerve may have some small overlap with the contralateral mental nerve. It also has some communication with the facial nerve.

The orifice of exit of the mental nerve, mental foramen, is most commonly located halfway between the alveolar crest and the inferior border of the mandibular bone in the vertical plane [6] and, in the horizontal plane, at the level of the apex of the second premolar or between the premolars [7].

Embryology

In general, nerves derive from the ectodermal germ layer, which further divides into neuroectoderm and neural crest cells. The nerves from the central nervous system develop from the neuroectoderm, while the peripheral nerves originate from the neural crest cells. Therefore, the trigeminal nerve and its branches, including the mental nerve, come from the neural crest cells.

The brachial apparatus gives rise to the trigeminal nerve in the face. These apparatus develop into the muscles, vessels, bone, and nerves in the face and neck. The brachial arch is made up mainly of mesoderm and neural crest cells. The first brachial arch develops into the mandibular nerve, which gives rise to the inferior alveolar nerve and consequently to the mental nerve.

Blood Supply and Lymphatics

The blood supply to the mental nerve and its sensory territory comes from four main arteries: inferior labial artery, mental artery, submental artery, and inferior alveolar artery. In addition, many anastomoses form the vascular network around the lips that provides collateral blood flow to the lips and chin region. The submental and the submandibular lymph nodes collect lymph from the sensory territory of the mental nerve. These lymph nodes will eventually drain back into the central circulation. The right side of the chin drains back into the central circulation via the right lymphatic duct, whereas the left side via the thoracic duct.

Muscles

The mental nerve only provides sensory innervation and lacks motor function. However, it has an anatomical association with the depressor anguli oris muscle. As the mental nerve exits the mental foramen, it divides into its three branches underneath or posterior to the depressor anguli oris muscle.

Physiologic Variants

The branching of the mental nerve may vary slightly. After exiting the mental foramen, it usually splits into three branches, but there may be more or fewer divisions in some individuals. More than one mental foramen may occur in around 1% of patients as a result of the mandibular canal bifurcation in the inferior-superior plane or medial-lateral plane [8][9]. However, the sensory territory is consistent [10][11].

Some individuals present an anatomical variation known as the anterior loop of the mental nerve. Some authors describe it as the extension of the inferior alveolar nerve before leaving the canal, anterior to the mental nerve; others define it as the mental neurovascular bundle that loops back to leave the mental foramen [7].

Surgical Considerations

Surgeons should know the mental nerve sensory territory and its foramen to perform a successful nerve block and prevent complications during surgical procedures. For example, in plastic surgery, the mental nerve is blocked during chin reconstructive interventions. As a result, in mentoplasties and other chin surgeries, the patient may be awake — the nerve block aids in safe and successful surgery with reduced potential complications. The surgeon may also block the mental nerve in the jaw and lower lip surgeries [12][13].

Nerve damage due to stretching, compressing, or transecting the mental nerve can cause sensory dysfunctions of the inferior lip, surrounding skin and mucosa, and teeth, including numbness, increased or decreased sensation, painful sensation, or complete loss of sensation. Many factors can produce such disturbances, for example, a denture compressing the nerve, a dental implant invading the nerve, edemas, hematomas, or dental injections. Furthermore, osteotomies can damage the inferior alveolar and mental nerve and blood vessels of the area [7].

Clinical Significance

The mental nerve block is mostly used in oral and dermatological procedures. It provides a better alternative to local infiltration when repairing lacerations of the skin of the inferior lip or chin, preventing edema and tissue distortion that may result in poor aesthetic appearance after suturing [13][14]. However, it is essential to know that a mental nerve block alone does not provide adequate anesthesia for dental work since it does not innervate the teeth [15][1][16].